The invention refers to the field of electronic reproduction technology and is directed to a method and to an arrangement for producing rastered color separations and printing forms for multi-color printing having rasters with arbitrary raster angles and raster widths.
In a color scanner for producing rastered color separations, three primary color signals are acquired by point-by-point and line-by-line opto-electronic scanning of a color original, these color signals being converted in a color computer into the color separation signals for recording the color separations "yellow", "magenta", "cyan" and "black" of a set of color plates. A point-by-point and line-by-line recording of the four rastered color separations of a set of color plates on a recording medium occurs after a processing of the color separation signals together with raster signals. The rastered color separations of a set of color plates serve as printing forms for the multi-color printing. The superimposed printing of the printing forms inked with the inks "yellow", "magenta", "cyan" and "black" occurs in a printing press on a print carrier to form a multi-colored reproduction.
Moire patterns can occur due to the superimposed printing of the raster dots, these having a particularly disturbing effect when viewing the finished print. The visibility of Moire patterns, as is known, can be diminished in that the rasters of the individual color separations of a set of color plates are printed on top of one another turned relative to one another, for which purpose the color separations must be recorded with rasters having different raster angles. What is achieved by the turned superimposed printing of the individual rasters is that the Moire periods are either too small or too big in order to be perceived in the print by the human eye as disturbing.
The rasters of the four color separations of a set of color plates must therefore be recorded with four different raster angles. In order to obtain a Moire minimum, the color separation "yellow" is often recorded in practice with a raster angle of 0.degree. , the color separation "magenta" is recorded with a raster angle of -15.degree. , the color separation "cyan" is recorded with a raster angle of +15.degree. and the color separation "black" is recorded with a raster angle of +45.degree.. These raster angles must be very strictly observed in the manufacture of the color separations, since disturbing Moire patterns can again occur, even given small angular deviations.
Raster angles other than those cited above are required when, for example, further colors are printed in addition to the four inks, when a different print medium is employed or when different screen widths are to be printed on top of one another.
German Patent 28 27 596 already discloses a method and an apparatus for producing color separations with rasters having arbitrary raster angling and raster width on the basis of point-by-point and line-by-line scanning of a color original and by point-by-point and line-by-line recording of raster dots with a recording element moving across a recording medium.
In the known method, a matrix is subdivided into a plurality of matrix elements and a raster threshold that corresponds to a tonal value is allocated to every matrix element. The raster thresholds of the matrix represent the periodically recurring basic structure for every raster mesh of the turned raster to be recorded (raster hill). The raster thresholds allocated to the individual matrix elements are deposited in a memory matrix at those respective memory locations which correspond to the position of the corresponding matrix elements within the matrix. The recording medium for the color separations is subdivided into a plurality of surface elements that are aligned in a recording direction of the recording element and transversely relative thereto. The raster dot within every raster mesh of the turned raster is composed of recorded surface elements, whereby the size of the raster dot corresponding to the tonal value to be reproduced is dependent on the plurality of exposed surface elements per raster mesh. During the recording of the rastered color separation, the raster element travels over the individual surface elements line-by-line and, dependent on the tonal values measured in the color original, a check is carried out for every surface element that is traversed to see whether it is to be exposed as part of a raster dot or not. In the check, the position of the momentarily traversed surface element within the raster mesh of the turned raster is identified in terms of coordinates and that raster threshold whose position within the matrix corresponds to the identified position of the traversed surface element in the corresponding raster mesh is called in on the basis of an appropriate addressing of the memory matrix.
The decision about a potential recording of the traversed surface element occurs by comparing the tonal value allocated to the corresponding raster mesh to the raster threshold that has been called in, whereby a recording signal is derived from the comparison which switches the recording element for recording the corresponding surface element on or not.
Although turned rasters having arbitrary raster angles and raster widths can be produced with the known method, disturbing Moire patterns can nonetheless occasionally occur.
Due to the topical quantization of the raster hill or, respectively, due to the finite bit width of the memory addresses, deviations in the raster geometry occur given raster angles that do not lead to whole-number memory addresses. When viewed over a great number of raster meshes, these deviations in fact cancel, but do lead to periodically repeating stripes that are disturbingly visible in the reproduction.
German Patent 29 29 876 already discloses a method for reducing disturbing patterns wherein random numbers are superimposed on the calculated memory addresses (address wobbling), this leading to arbitrarily modified raster dot shapes. Disturbing patterns cannot be entirely avoided with this method either in extreme cases.